Christoph Hellwig <hch@xxxxxxxxxxxxx> writes: > On Thu, Aug 13, 2020 at 05:11:20PM +1000, Daniel Axtens wrote: >> When returning results to userspace, do_sys_poll repeatedly calls >> put_user() - once per fd that it's watching. >> >> This means that on architectures that support some form of >> kernel-to-userspace access protection, we end up enabling and disabling >> access once for each file descripter we're watching. This is inefficent >> and we can improve things by batching the accesses together. >> >> To make sure there's not too much happening in the window when user >> accesses are permitted, we don't walk the linked list with accesses on. >> This leads to some slightly messy code in the loop, unfortunately. >> >> Unscientific benchmarking with the poll2_threads microbenchmark from >> will-it-scale, run as `./poll2_threads -t 1 -s 15`: >> >> - Bare-metal Power9 with KUAP: ~48.8% speed-up >> - VM on amd64 laptop with SMAP: ~25.5% speed-up >> >> Signed-off-by: Daniel Axtens <dja@xxxxxxxxxx> > > Seem like this could simply use a copy_to_user to further simplify > things? I'll benchmark it and find out. > Also please don't pointlessly add overly long lines. Weird, I ran the commit through checkpatch and it didn't pick it up. I'll check the next version more carefully. Regards, Daniel
